Millimeter-wave monolithic diode-grid frequency multiplier

ABSTRACT

A semiconductor diode structure useful for harmonic generation of millimeter or submillimeter wave radiation from a fundamental input wave is fabricated on a GaAs substrate. A heavily doped layer of n ++  GaAs is produced on the substrate and then a layer of intrinsic GaAs on said heavily doped layer on top of which a sheet of heavy doping (++) is produced. A thin layer of intrinsic GaAs grown over the sheet is capped with two metal contacts separated by a gap to produce two diodes connected back to back through the n ++  layer for multiplication of frequency by an odd multiple. If only one metal contact caps the thin llayer of intrinsic GaAs, the second diode contact is produced off the diode structure and connected to the n ++  layer for multiplication of frequency by an odd multiple. If only one metal contact caps the thin layer of intrinsic GaAs, the second diode contact is produced to connect to the n ++  layer for multiplication of frequency by an even number. The odd or even frequency multiple is selected by a filter. A phased array of diodes in a grid will increase the power of the higher frequency generated.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract, and is subject to the provisions of Public Law 96-517(35 USC 202) in which the Contractor has elected not to retain title.

TECHNICAL FIELD

This invention relates to a millimeter-wave monolithic diode-gridfrequency multiplier using barrier-intrinsic-n+ (BIN) semiconductordiodes useful for harmonic generation of mixing microwave signals atmillimeter and submillimeter wavelengths in, for example, submillimeterwave spectroscopy.

BACKGROUND ART

A serious problem in the development of submillimeter wavelengthreceivers for atmospheric and space spectroscopy is the availability ofsuitable local oscillators required for heterodyne mixing. Such localoscillators must have reasonable power output and efficiency, and arerequired to cover a range of wavelengths of interest to spectroscopy.Lasers are being developed for this purpose, but each laser system isrestricted essentially to a single wavelength. Some tunability can beachieved by optical techniques, but only over very limited bandwidths.Microwave generators such as carcinotrons do not operate efficiently atwavelengths shorter than a millimeter, and they are excessively heavy,power consuming and of short lifetime, restricting their use in flightmissions. Available solid-state oscillators, such as GaAs Gunn diodesand IMPATT (impact ionization avalanche transit time) diodes are highlyefficient and tunable, but are currently limited to frequencies up toabout 75 and 150 GHz, respectively, for output power ≧0.1W.

Much higher frequencies can be obtained by generating harmonics of thefundamental frequency from these available solid-state oscillators. Twocascaded harmonic multipliers based on whisker-coupled GaAs varactordiodes in waveguide configurations have produced 0.3 mW at 492 GHz.However, waveguide fabrication and impedance matching technologies arealready at their limits at this frequency. Therefore, planar structuresfor quasi-optical coupling are preferred. In the planar technology,arrays of diodes can be easily fabricated and integrated with antennastructures. The total power produced is then proportional to the numberof diodes provided. However, varactor diodes have serious limitations athigher frequencies. These stem primarily from the parasitic resistanceintroduced by the front ohmic contact. Furthermore, the weak dependenceof the capacitance on the voltage C(V) limits efficiency in harmonicgeneration, especially for higher harmonics.

Another example of this planar array approach, which overcomes thedeficiencies of the varactor diodes involves T-MOS (thinmetal-oxide-silicon) diodes, which have an undoped thin epitaxialsilicon layer, and exhibit an exponential dependence of the space chargecapacitance on voltage, and thus produces harmonics more efficiently.This has been demonstrated with single diodes in a whisker-coupledwaveguide configuration for frequency doubling and tripling.Furthermore, due to the blocking barrier, two diodes can be operatedback-to-back generating a sharp spike in the C(V) curve. Thisarrangement, which needs no external ohmic contact, makes a highlyefficient frequency tripler in which the efficiency does not degradewith high fundamental power. As a further advantage, the input andoutput impedances are doubled. However, defects in the epitaxial siliconlayer deteriorate the thin oxide and limit the fabrication yield of thedevice.

Yet another planar array approach investigated involves monolithicSchottky diode grids fabricated on 2-cm square gallium-arsenide wafers.Second harmonic conversion efficiencies of 9.5% and output powers of 0.5W were achieved at 66 GHz when the diode grid was pumped with a pulsedsource at 33 GHz. However, using currently realizable diode parameters,it should be possible to achieve CW doubling efficiencies of 60% at 66GHz with output powers of 2 W using edge cooled wafers.

STATEMENT OF THE INVENTION

A millimeter or submillimeter wave monolithic diodegrid frequencymultiplier is provided on a substrate wafer of a semiconductor materialhaving high electron mobility and saturation velocity. The structureconsists of a grid of metal that functions as an antenna connected to anarray of semiconductor diodes, each comprised of a heavily doped n++layer of semiconductor material functioning as a low resistance backcontact, an intrinsic layer of the semiconductor material, and a barrierbetween the intrinsic layer and at least one metal contact, that barriercomprising a sheet of n++ doping over the intrinsic layer and a thinbarrier layer between that sheet and at least one metal contact on aplanar surface of the structure. If just one of two metal contacts isdeposited on the barrier layer, a single diode for frequencymultiplication by an even number results, and if both of the metalcontacts are deposited on the barrier layer with a gap between the metalcontacts, two diodes connected back to back through the heavily dopedn++ layer results for multiplication by an odd number.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionwill best be understood from the following description when read inconnection with the accompanying drawings

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a illustrates schematically a submillimeter-wave frequencymultiplier array of barrier-intrinsic-n+ (BIN) diodes, and FIG. 1billustrates in greater detail two BIN diodes shown schematicallyconnected back to back in the structure of FIG. 3.

FIG. 2 illustrates schematically one half of a longitudinally sectionedpair of BIN diodes fabricated on a semiintrinsic GaAs substrate andconnected back to back for frequency tripling.

FIG. 3 is a schematic diagram of a single BIN diode fabricated on asemi-intrinsic GaAs substrate for frequency doubling.

FIG. 4 is a diagram of the energy and doping profile of the BIN diodeshown in FIG. 3.

FIG. 5 is a graph illustrating the inverse capacitance dependence of asingle BIN diode, and of two back-to-back BIN diodes.

DETAILED DESCRIPTION OF THE INVENTION

In order to achieve an inexpensive watt level CW solid-state source ofradiation for millimeter and submillimeter wave applications, thepresent invention illustrated schematically in FIG. 1 provides asemiconductor BIN diode grid 10 for frequency multiplication. Forsimplicity of illustration, a 5×5 grid of conductors 10a is shown withBIN diodes 10b. Millimeter-wave radiation from a source, such as anIMPATT diode (not shown), is passed through a filter 11a onto the array10 of BIN diodes 10b monolithically integrated on a gallium-arsenidewafer 10c typically 2-cm square. There radiation is multiplied by thediodes in the array to increase the input frequency from f to kf, wherek may be an even integer for the case of single BIN diodes, or an oddinteger for the case of two back-to-back BIN diodes. For example, in thecase of back-to-back diodes, the input frequency may be tripled fromabout 33 GHz to about 99 GHz, with an efficiency of greater than 35%from the input to the output radiation. The output radiation is passedthrough a filter 11b which selects the tripled frequency from among allof the odd multiples generated.

The present invention uses selective planar doping during growth bymolecular beam epitaxy (MBE) to define a BIN (barrier-intrinsic-n+)diode structure. Two possible device configurations are illustrated inFIGS. 2 and 3. FIG. 2 illustrates a back-to-back configuration of twoBIN diodes which would function as an efficient frequency multiplier inwhich the even multiples are cancelled, and FIG. 3 illustrates a singleBIN-diode which would function as a frequency multiplier which generatesonly even multiples. Because the two configurations are so similar, thesame reference numerals are applied to both FIG. 2 and FIG. 3 to referto the same elements in each configuration.

Referring to FIG. 2, this structure does not require an insulator layer,as in thin-MOS (T-MOS) diode structures, but instead relies on aMott-type barrier formed by an intrinsic layer in the semiconductorstructure between metal contacts 14a and 14b (which define a pair ofback-to-back diodes) and an n++ sheet 16 of positive charge formed by adoping (n++). It should be understood that the barrier may also beformed in other ways, such as by an insulating oxide layer or by asemiconductor material with wider bandgap, e.g., by heteroepitaxy.

The active region for nonlinear response and multiplication of inputradiation is an intrinsic layer 18 between the barrier layer 12 and aheavily doped n++ layer 20. Accumulation and depletion of electrons atthe barrier layer 12 by space-charge-limited current producenonlinearities in the capacitance-voltage characteristic which are muchstronger than in a varactor diode, especially at low temperatures. Alsoin each BIN diode the RC product is minimized for maximum cut-offfrequency. The device structure can be readily grown by molecular beamepitaxy MBE with semiconductor materials having high mobility andsaturation velocity, such as GaAs (or even more so InAs), furtherextending the cut-off frequency.

The maximum cut-off frequency is determined by the time it takeselectrons to transit the space charge layer 20 at saturation velocity.For a 100-nm n++ GaAs layer, the cut-off frequency f_(c) ≈1 THz. Anefficiency of ˜ 1% is calculated for f_(out) =f_(c). The efficiency forf_(out) =0.3 f_(c) is ˜ 10%. A combined efficiency of 0.1% is projectedfor f_(out) =1 THz for two back-to-back diodes shown in FIG. 2. Hence,100 mW input power is required for the projected minimum output power of100 μW, a goal presently achievable using an IMPATT diode as a source.The planar growth process (e.g., MBE) also lends itself to fabricationof other integrated circuitry on the same wafer by standardphotolithographic techniques, such as integration with optimizedcoupling structures for quasi-optical coupling of submillimeter waveradiation. An output power of 1 mW can be achieved with very smallarrays of ˜ 10 diodes. Larger arrays of ˜ 100 diodes can be accommodatedon commonly sized chips for higher output power; however, a larger BINdiode array requires a proportionally stronger source, e.g., aphase-locked array of IMPATT diodes.

FIG. 3 illustrates a single BIN diode fabricated in place of a pair ofback-to-back diodes. By comparing FIGS. 2 and 3, it may be readilyappreciated that the only difference in the structures is that thebarrier layer 12, intrinsic layer 18 and n++ sheet 16 are terminated inthe gap at or before the point where the second metal contact 14bbegins. In place of the layer 18 and barrier 12, and the n++ sheet 16therebetween, a layer 21 is provided to even the planar surface for thecontact 14b and provide a low resistance connection of the metal contact14b to the heavily doped n++ layer 20. This may be provided by MBEgrowth of the layer 21 heavily doped n++, or by deposition of an alloy(such as Au, Ge, Ni). Yet another technique that may be employed isimplanting charges in the layer of intrinsic GaAs not etched beforedepositing the metal gate 14b.

FIG. 4 illustrates the doping profile and energy (conduction band edge)versus position z (growth direction) for fabrication of a single BINdiode on a GaAs substrate. The doping profile can be achieved entirelyduring epitaxial growth starting with a semi-insulating (s.i.) GaAssubstrate. The 2.4 μm n³⁰ + layer 20 provides the back contact betweenback-to-back diodes and between a single diode and contact 14b. The100-400 nm intrinsic (i) or undoped layer 18 provides thespace-charge-limited active region, and the n++ sheet 16 of dopant (≅2×10¹² cm⁻²) terminates the Mott barrier formed between the intrinsiclayer 18 and the metal (Al) contact 14. The n++ sheet 16 should be asthin as possible (e.g., <2 nm) for optimum performance. The thickness ofthe barrier layer 12 is about 30 nm.

The graph of FIG. 5 shows the inverse capacitance dependence of eachsingle BIN diode (dashed curves) of the two back-to-back BIN diodes andtheir combined series effect (solid curve) due to thespace-charge-limited capacitance C_(SCL) of the intrinsic layer 18. (Thecapacitance of the barrier layer 12 is large and its inverse value issubtracted out for convenience.) C_(i) is the assympotic value ofC_(SCL) for strong reverse bias and is equal to the geometriccapacitance of the intrinsic layer 18 (εA/w_(i)). These curves werecalculated from the relations derived by D.P. Howson, et al., SolidState Electronics, 8, 9 3 (1965).

The total inverse capacitance (of the back-to-back diodes) shown as thesolid curve assumes a bias of 6kT/e. The height and width of this curvecan be adjusted with a dc bias (applied to the n++ region, layer 20) foroptimum performance. The curve shown is considered near optimum.However, this optimum condition can be achieved much more convenientlyby doping control alone of the n++ sheet 16 creating the desired offsetvoltage (flat band voltage) and thus eliminating the need for anexternal dc bias. The narrow width of the C-V dependence (e.g., 6kT/e)provides for efficient multiplication even at low input power andefficient high order multiplication (e.g., 5, 7, 9 . . . at high inputpower. With planar doping techniques the resistivity of the n++ backcontact (layer 20) can be pushed into the 10¹⁹ /cm³ region. Theparasitic RC product then becomes an order of magnitude smaller than thetransit time. A tradeoff between the maximum cut-off frequency and themaximum change of capacitance for efficient multiplication is therebyachieved by simply adjusting the thickness of the intrinsic layer 18.

In each device configuration shown in FIGS. 2 and 3, a mesa structure isetched out (by ion beam or chemical etching) to define the active areaof one element (BIN diode or backto-back BIN diodes) of an array. Theetched regions 22 may be filled with polyimide to provide a planarsurface for subsequent Al metallization of the contacts 14a and 14b(FIGS. 2 and 3). These metal contacts may be extended to interconnect anarray of BIN diodes in a pattern for quasi-optical coupling to radiationas shown in FIG. 1. An alternate technique to etching and filling withpolyimide would be to proton bombard the area around the BIN diodes toconvert it to semi-insulating GaAs. This technique is easier to apply,but may cause some degradation of the device (e.g., lower breakdownvoltage).

Although particular embodiments of the invention have been described andillustrated herein, it is recognized that modifications and variationsmay readily occur to those skilled in the art, particularly in themanner in which the Mott-type barrier is provided. However, theembodiments illustrated have particular advantages, namely the use ofuniform material with just selective dopied, and the ability to tailorflat band voltage for optimum performance. Consequently, it is intendedthat the claims be interpreted to cover such modifications andvariations.

What is claimed is:
 1. A semiconductor diode structure useful forharmonic generation of millimeter or submillimeter wave radiation from afundamental input wave comprisinga substrate, a n++ doped layer ofsemiconductor material on said substrate, a layer of intrinsicsemiconductor material on said n++ doped layer, a sheet of positivecharge firmed by surface n++ doping said intrinsic layer on the surfacethereof opposite said n++ doped layer, and, a Mott-type barrier formedover said sheet of n++ doping by a layer of material electricallyinsulating said sheet from at least one of a pair of surface metalcontacts, at least one of said surface contacts being deposited oversaid layer of electrically insulating material with a gap between saidsurface contacts.
 2. A semiconductor diode structure as defined in claim1 wherein only one of said surface metal contacts is deposited over saidlayer of insulating material and the other is not, and means for makingan electrical connection of said other metal contact to said n++ dopedlayer of semiconductor material.
 3. A semiconductor diode structure asdefined in claim 1 wherein both of said metal contacts are depositedover said layer of insulating material, one over each half of said layerof insulating material.
 4. A semiconductor diode structure as defined inclaim 1 wherein said layer of insulating material is selected fromintrinsic semiconductor material, an oxide of semiconductor material ora different semiconductor material with a wider band-gap than saidsemiconductor material.
 5. A semiconductor diode structure as defined inclaim 4 wherein said substrate is GaAs and said n++ doped layer ofsemiconductor material is GaAs.
 6. A semiconductor diode as defined inclaim 5 wherein said layer of insulating material is intrinsic GaAs. 7.A semiconductor diode as defined in claim 6 wherein said pair of surfacemetal contacts are electrically connected to conductors functioning asan antenna.
 8. A millimeter or submillimeter wave monolithic diode-gridfrequency multiplier comprising a substrate of semiconductor material, agrid of metal that functions as an antenna, and a phased array of diodesconnected to said grid of metal for receiving radiation at one frequencyand producing radiation through said antenna at a higher frequency thatis a multiple of the radiation frequency received, each diodecomprisinga n++ doped layer of semiconductor material on said substrate,a layer of intrinsic semiconductor material on said n++ doped layer, asheet of positive charge formed by surface n++ doping said intrinsiclayer on the surface thereof opposite said n++ doped layer, a Mott-typebarrier formed over said sheet of n++ doping by a layer of materialelectrically insulating said sheet from at least one of a pair ofsurface metal contacts, said surface metal contacts being deposited witha gap therebetween, and said surface metal contacts connecting saiddiode in series in a unique branch of said grid not shared by otherdiodes, with at least one metal contact of each diode over its layer ofinsulating material, and means for isolating each diode structure fromall other diode structures on said substrate with only said gridinterconnecting said phase array of diodes.
 9. A monolithic diode-gridfrequency multiplier as defined in claim 8 wherein only one of saidsurface metal contacts of each diode is over said layer of insulatingmaterial and the other is not, and means for low-resistance connectionof said other surface metal contact of each diode to said n++ dopedlayer of semiconductor material in its isolated structure.
 10. Amonolithic diode-grid frequency multiplier as defined in claim 8 whereinboth of said metal contacts of each diode are over said layer ofinsulating material, one over each half of said barrier with a gapbetween said one and said other contact.
 11. A monolithic diode-gridfrequency multiplier as defined in claim 10 wherein said layer ofinsulating material is selected from intrinsic semiconductor material,an oxide of semiconductor material or a different semiconductor materialwith a wider bandgap than said semiconductor material.
 12. A monolithicdiode-grid frequency multiplier as defined in claim 11 wherein saidsubstrate is GaAs and said heavily doped layer of semiconductor materialis n++ GaAs.
 13. A monolithic diode-grid frequency multiplier as definedin claim 12 wherein said thin layer of material electrically insulatingsaid sheet of n++ doping from said metal contact is intrinsic GaAs.